Postgres 12

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

create table t (id int primary key, info text);

CREATE TABLE

insert into t values (1, 'old');

INSERT 0 1

insert into t 
select n, 'new'
from generate_series(1, 5) as n(n)
on conflict do nothing;

INSERT 0 4

select * from t;

id
info

1
old

2
new

3
new

4
new

5
new

SELECT 5